An optical-heterodyne alignment technique for quarter-micron x-ray lithography

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Abstract
A new optical‐heterodyne interferometry alignment technique with diffraction gratings is developed for quarter‐micron x‐ray lithography. To obtain detection accuracy as good as a few tens of nanometers, a phase signal is utilized instead of a conventional intensity signal. The relative lateral displacement between mask and wafer is detected by measuring the phase difference between heterodyne beat signals generated by projecting two laser beams from + first‐order and − first‐order diffraction directions on the mask and wafer grating marks. The displacement signal is only slightly influenced by gap variation using symmetric optics. A lateral displacement detection resolution better than 10 nm is obtained by the experimental alignment setup. A nonsymmetric beam from the − third‐order diffraction direction is added to the symmetric beams to detect the gap. The phase difference between two beat signals emitted to the second‐order diffraction direction from the same mask and wafer marks is used as the gap detection signal. The cyclic gap signal makes it possible to set an arbitrary gap. A gap detection resolution of <20 nm is realized. Using this optical‐heterodyne interferometry alignment method, a four‐channel alignment system is developed for synchrotron x‐ray lithography. Six‐axis alignment servo control is established by combining this system with highly accurate stages.

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